1. Field of the Invention
The present invention relates to a drawn cup needle roller bearing.
2. Description of the Background Art
Conventionally, as a bearing used in a transmission, a compressor for a car air conditioner, a stator and the like, a drawn cup needle roller bearing has been used because it can receive high load although its projected area is small, and its cost is low. Such drawn cup needle roller bearing comprises a drawn cup, a plurality of rollers arranged along the inner diameter surface of the drawn cup on which the rollers rotate, and a retainer for retaining the plurality of rollers.
Here, a manufacturing method of a conventional drawn cup needle roller bearing will be briefly described. FIG. 6 is a schematic view showing an example of a manufacturing method of the conventional drawn cup needle roller bearing. Referring to FIG. 6, first, a flat steel plate 101 that is the material of a drawn cup is formed into a shape of a cup by a deep drawing process and the bottom of the cup is punched out and one flange 102 is formed. Then, a treatment for a rolling surface 103 is performed and a quenching treatment such as a carburized quenching treatment is performed for the whole drawn cup. Here, since the quenching treatment is performed once after the bottom of the cup has been punched out, a carburization proof treatment or an annealing treatment is to be performed so that an edge part 104 may not be cracked at the time of bending before a flange 107 is bent at an entrance of the cup. Then, rollers 105 and a retainer 106 are incorporated and the edge part 104 is bent and the other flange 107 is formed. Thus, the drawn cup needle roller bearing is completed through a step of cleaning and the like.
As the material of the roller to be incorporated, JIS SUJ steel is used in general. The roller contains retained austenite that is gradually reduced from its surface layer to the inside and its amount is 15% by volume at most in general. As a result, Vickers hardness (Hv) value of the surface of the roller is about 700 to 750. In addition, as the material of the drawn cup, case-hardening steel such as JIS SCM415 steel is used, and as the material of the retainer to be incorporated, normal cold-rolled steel (JIS SPCC steel, for example) is used and soft-nitrided to improve the hardness thereof.
According to the drawn cup needle roller bearing manufactured as described above, since the carburization proof treatment or the annealing treatment is performed at the edge part 104, the hardness of the edge part 104 is low, so that the drawn cup has uneven hardness. Furthermore, since a heat treatment history is different, there is a variation in diameters at both ends where the flanges are bent and diameter of a center part, so that its roundness cannot be preferable. As a result, a long life cannot be implemented.
In view of the above problems, Japanese Unexamined Patent Publication No. 10-46318 discloses a manufacturing method in which after rollers and a retainer are incorporated, an edge part 104 is bent and then an entire drawn cup needle roller bearing comprising the rollers and the retainer is heat treated.
According to the Japanese Unexamined Patent Publication No. 10-46318, since the entire drawn cup needle roller bearing is heat treated after the rollers and the retainer have been incorporated and the edge has been bent, an annealing treatment and the like is not necessary. Therefore, its roundness is preferable and the hardness of the drawn cup does not become partially low. Thus, a long life can be implemented. In addition, since the heat treatment process can be simple, the drawn cup needle roller bearing can be manufactured at low cost.
According to the drawn cup needle roller bearing manufactured by the above method, since a long life and low cost can be implemented, it can be used in various kinds of usages. For example, it can be used as a bearing that supports a throttle shaft in a throttle valve machine that requires a small bearing projected area and low cost. Here, when it is used as the bearing that supports the throttle shaft, it is required that the bearing is superior in sealing property, that is, fluid leakage preventing function.
A seal to be incorporated to prevent the fluid leakage includes a seal comprising a metal core. However, since the metal core is rigid, it is difficult to change its configuration, so that it cannot be incorporated inside the drawn cup after the bending process in the manufacturing process disclosed in the Japanese Unexamined Patent Publication No. 10-46318. Therefore, it is necessary to incorporate it inside the drawn cup before the bending process is performed. However, since the seal lacks in durability at a high temperature, when it is heat treated under the same condition as that of the drawn cup and the like at the heat treatment process, the seal is considerably damaged. Thus, the low cost, long life and high sealing property cannot be implemented at the same time.